The invention is directed to a method and apparatus for preheating glass and raw materials containing organic materials in an indirect heat exchanger, which has at least one preheating shaft and an adjacent heating chamber, to temperatures up to a maximum just below the softening temperature of the glass, for subsequent charging into a glass melting furnace, by passing the charging material through at least one preheating shaft and by heating the heating chamber with waste gases from the glass melting furnace.
In the case of indirect heat exchangers, the heat emitting and the heat absorbing media are separated from one another in order to prevent mixing.
In conjunction with glass melting furnaces it is also known that the raw material, which includes glass forming materials, the so-called batch, and cullet from recycled glass, can be heated in indirect heat exchangers by the waste gases from the melting furnace, for example in order to avoid the transfer of unwanted components present in waste gases to the raw material. This method does not lead to problems as long as the raw material only contains water. The batch normally contains approximately 4% water, the cullet approximately 1-1.5% water. The water is normally released from the raw material in the form of water vapor, which can be discharged to the surroundings without any particular problems. In fact, unless specific steps are taken this cannot be prevented, as the heat exchanger has openings at the inlet and outlet points for the raw material, through which the water vapor can exit.
However, preheating of raw material with recycled glass causes problems if the recycled glass contains organic substances or contaminant materials, the previous removal of which is either difficult or at least very expensive, and which undergo a carbonization process during preheating to the usual preheat temperatures, which leads to the formation of gases and vapors, which must not be released into the atmosphere because of environmental protection regulations.
These organic substances include paper, plastics and adhesives from labels, plastic coatings on bottles, plastic bottle tops, and also contamination resulting from the original contents. Also, corks are included as an organic contaminant material.
For economic reasons the raw material should be preheated to as high a temperature as feasible, if possible to just under the temperature at which the raw material tends to agglomerate. If glass cullet is present, this process begins above the softening temperature of the glass, so that the preheating is normally completed at a maximum temperature of approximately 550.degree. C.
At such temperatures however a carbonization process takes place, in which non-filterable gaseous or vaporous compounds such as carbon monoxide, organic cracking products, dioxines, furanes and the dangerous halogenated hydrocarbons are formed. If no specific steps are taken, these gases and vapors are also emitted from an indirect heat exchanger, as a chimney effect occurs in the type of preheating shaft normally used.
The German patent DE-PS 2,932,052 teaches that is possible to introduce glass raw materials and waste glass containing harmful organic materials into the glass melt of a glass melting furnace at various locations, and to control the furnace atmosphere so that there is an oxidizing atmosphere with high temperatures at the locations at which the waste glass with the organic materials is introduced, so that the organic materials are incinerated in the glass melting furnace. However, the maintenance of stable operation is complicated, and the formation of carbon-rich cords in the glass melt cannot be avoided, which leads to unusable end products. The energy consumption of such a melting process is also unsatisfactory, as the recycled glass, which normally forms the largest proportion of the material introduced into the furnace, is not preheated. This therefore concerns a non-generic process.
U.S. Pat. No. 5,156,545 describes a further non-generic process, the main objective of which is the use of a pyroytic process for the treatment of glass fibers that are contaminated with organic materials, so that the fibers can be re-melted in a glass melting furnace. This is not a preheating process, as it is permissible to dump the end product. The pyrolytic process is carried out on loose piles of material, which are passed through a combustion chamber on a conveyor belt. In this case the material to be processed also has a tendency to stick, and so the temperature must be limited to prevent sticking, which means that the temperature must be kept below 550.degree. C. The only information given about the gas required for the pyrolysis is that it should contain less than 10% oxygen, in order to avoid local high temperatures, which could cause the material to stick. Since this means that full pyrolysis is not possible, it is stated that the waste gases from the process should be passed through gas treatment equipment and, if necessary, they should be recirculated. It is also stated that the pyrolysis gases should be transported to a special chamber for incineration. A combination of the energy balance of the pyrolysis process with that of the glass melting furnace is apparently not taken into consideration. Process operation, and the necessary equipment are complicated and expensive, and the method is therefore only suitable for special waste products, not for a normal glass melting process.